In the 1970s and 1980s all inclusive holidays from MAN to the Med could be as much as 25% more than from Luton or Gatwick depending on the season and timing.
Also I had an aunt who regularly flew Wardair to Canada. She quickly learned that it was cheaper to fly from Gatwick than from MAN (and face it, Wardair did very well ex MAN) but was most annoyed to find one time when she had had to visit her daughter in Croydon before heading to Toronto, that the cheaper flight from Gatwick stopped at MAN on the way and, when she tried to change her return booking to get off at MAN they wanted to charge her the difference between the two fares plus a handling charge.
Airline fares are mad. I' m on the last leg of a round the world trip. The booking from LHR back to Gatwick was originally for eleven sectors Qantas added two more by cancelling the Alice Springs to Cairns and rerouting us, business class via Sydney and Brisbane. The original routing was Heathrow, Hong Kong, Singapore, Melbourne, Alice Springs, Cairns, Sydney, Auckland, Rarotonga, back to Auckland, Los Angeles and, after a road trip across the US, Orlando to Gatwick. The cost, departing in early February in economy was £2,200 inc all taxes, full travel/medical insurance at double rates as I'm over 65 and an Australian visa. If I had added the Shannon Heathrow one way sector which cost me £60 to buy online it would have added another £120 as I'm terminating the trip in London.
A cousin flew out to Auckland and back over Singapore, no stops permitted. Cost to him, also in economy, £895. Where is the logic?